docs: def url(context, view, subdomain=UNSET, *args, **kwargs):
      """
      Resolves a URL in a template, using subdomain-based URL resolution.
  
      If no subdomain is provided and a ``request`` is in the template context
      when rendering, the URL will be resolved relative to the current request's
      subdomain. If no ``request`` is provided, the URL will be resolved relative
      to current domain with the ``settings.ROOT_URLCONF``.
  
      Usage::
  
          {% load subdomainurls %}
          {% url 'view-name' subdomain='subdomain' %}
  
      .. note:: This tag uses the variable URL syntax introduced in Django
         1.3 as ``{% load url from future %}`` and was made the standard in Django
         1.5. If you are upgrading a legacy application from one of the previous
         template tag formats, make sure to quote your constant string URL names
         to avoid :exc:`~django.core.urlresolver.NoReverseMatch` errors during
         template rendering.
  
      """
      if subdomain is UNSET:
          request = context.get('request')
          if request is not None:
              subdomain = getattr(request, 'subdomain', None)
          else:
              subdomain = None
      elif subdomain is '':
          subdomain = None
  
      return reverse(view, subdomain=subdomain, args=args, kwargs=kwargs)